Patent GB-887574-A (GB)
Title: Improvements in or relating to electric wave filters
Inventor: WARE DAVID GEORGE
IPC: H01P1/213
Filed: 1960-01-22
Abstract: 887,574. Waveguide filters. STANDARD TELEPHONES & CABLES Ltd. Jan. 22, 1960, No. 2391/60. Class 40(8). An electric wave filter for separating or combining electric waves operating in the TE 10  mode and having frequencies lying in separated frequency bands comprises a main circular, square or half-square waveguide 1 adapted to transmit waves having frequencies lying in both frequency bands, a secondary ridge waveguide or waveguides 2, 3 arranged with its axis or axes parallel to the axis of the main waveguide, a plurality of branching rectangular guides 5 connecting the main waveguide and the secondary waveguide(s), the branching guides having their cut-off frequency lying between the frequency bands and being arranged with their central H-planes perpendicular to the axes of the main and secondary guides and spaced apart at substantially equal intervals, and reflectionless terminations at the upper ends of the secondary guides 2, 3. The secondary guides 2, 3 can have the same cut-off frequency as the main guide 1. The lower ends of the secondary guides 2, 3 are coupled to rectangular extension guides 6, 7 by transition sections of the guides 2, 3 in which the height of the ridge 4 is reduced substantially to zero in a series of steps 13. The two guides 6, 7 are coupled to an output rectangular guide 9 by an E-plane T-joint 8. The guides 6, 7 have chamfered E-bends 10, 11 and chamfered H-bends 12. The branching guides 5 are spaced so that the distance between the central planes normal to the electric vector of adjacent guides is from one-eighth to three-eighths of a wavelength, preferably a quarter of a wavelength in the guide at the mid-band frequency of the upper band, and the lengths of the guides 5 are of similar magnitude or up to between fiveeighths and seven-eighths of a wavelength, preferably three-quarters of a wavelength. The branching guides 5 can all have the same cross-section or the heights of the guides at the ends of a set are less than those in the centre. Waves entering the guide 1 at the top end are separated into the two frequency bands issuing from the guide 9 and the lower end of the guide 1 respectively. If waves occupying the lower frequency band are supplied to the lower end of the guide 1 and waves occupying the upper band to the guide 9 they will be combined and issue from the upper end of the guide 1. When the guide 1 is of half-square section only one of the guides 2, 3 and one set of the guides 5 is required. In the construction shown in Fig. 6 two additional secondary ridge guides 15, 16 are coupled to the two remaining sides of the square main guide 1 by two sets of branching guides 17, 18 and to an output guide 19 by extension guides 20, 21 and a T-joint 22. If the guide 1 is supplied with two groups of waves polarised at right angles the waves of the upper frequency band having the electric vector normal to the line 14 will emerge from the guide 9, while those polarised with the vector parallel thereto will emerge from the guide 19. The two groups of waves need not occupy the same frequency bands-the guides 17, 18 can be differently spaced and proportioned from the guides 5. Reference has been directed by the Comptroller to Specification 820,632.